Plasma addressed liquid crystal display with glass spacers

ABSTRACT

A flat display device, preferably of the PALC type, in which the plasma channels are formed by etching laterally-spaced slots in a spacer plate, attaching a thin dielectric sheet over the etched spacer plate, and bonding the etched spacer plate to a transparent substrate such that each channel is formed by the portion of the substrate between flanking walls formed by the etched slots in the spacer plate, adjacent flanking walls in the spacer plate, and the overlying portion of the thin dielectric sheet. In a modification, strengthening crossbars are formed between adjacent flanking walls.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser No. 08/588,800, filed Jan. 19,1996 now U.S. Pat. No. 6,433,471.

RELATED APPLICATIONS

1) application Ser No. 08/384,090, filed Feb. 6, 1995, now U.S. Pat. No.5,914,562, issued Jun. 22, 1999.

2) application Ser No. 08/413,052, filed Mar. 29, 1995, now U.S. Pat.No. 5,596,431, issued Jan. 21, 1997.

3) application Ser No. 08/573,742, filed Dec. 18, 1995, now U.S. Pat.No. 5,764,001, issued Jun. 9, 1998.

BACKGROUND OF INVENTION

This invention relates to plasma channels, to display devices comprisingplasma channels, and to plasma-addressed liquid crystal display panelscommonly referred to as “PALC” display devices using such channels. PALCdevices comprise, typically, a sandwich of: a first substrate havingdeposited on it parallel transparent column electrodes, commonlyreferred to as “ITO” columns or electrodes since indium-tin oxides aretypically used, on which is deposited a color filter layer; a secondsubstrate comprising parallel sealed plasma channels corresponding torows of the display crossing all of the ITO columns and each of which isfilled with a low pressure ionizable gas, such as helium, neon and/orargon, and containing spaced cathode and anode electrodes along thechannel for ionizing the gas to create a plasma, which channels areclosed off by a thin transparent dielectric sheet; and a liquid crystal(LC) material located between the substrates. The structure behaves likean active matrix liquid crystal display in which the thin filmtransistor switches at each pixel are replaced by a plasma channelacting as a row switch and capable of selectively addressing a row of LCpixel elements. In operation, successive lines of data signalsrepresenting an image to be displayed are sampled at column positionsand the sampled data voltages are respectively applied to the ITOcolumns. All but one of the row plasma channels are in the de-ionized ornon-conducting state. The plasma of the one ionized selected channel isconducting and, in effect, establishes a reference potential on theadjacent side of a row of pixels of the LC layer, causing each LC pixelto charge up to the applied column potential of the data signal. Theionized channel is turned off, isolating the LC pixel charge and storingthe data voltage for a frame period. When the next row of data appearson the ITO columns, only the succeeding plasma channel row is ionized tostore the data voltages in the succeeding row of LC pixels, and so on.As is well known, the attenuation of the backlight or incident light toeach LC pixel is a function of the stored voltage across the pixel. Amore detailed description is unnecessary because the construction,fabrication, and operation of such PALC devices have been described indetail in the following U.S. patents and publication, the contents ofwhich are hereby incorporated by reference: U.S. Pat. Nos. 4,896,149;5,077,553; 5,272,472; 5,276,384; and Buzak et al., “A 16-Inch Full ColorPlasma Addressed Liquid Crystal Display”, Digest of Tech. Papers, 1993SID Int. Symp., Soc. for Info. Displ. pp. 883-886.

A partial perspective view of the PALC display described in the 1993 SIDDigest is shown in FIG. 2. The method described in the referencedpublication for making the plasma channels is to chemically etch a flatglass substrate to form parallel semi-cylindrically shaped recessesdefined by spaced ridges or mesas and to bond on top of the mesas a thindielectric cover sheet having a thickness in the range of about 30-50μm.

The above construction and its fabrication encounters certain problems.Since the channel electrodes must be patterned on the sloping sidewallof the channel, the dimensions and placement of the electrodes cannot beaccurately controlled. Moreover, since slight variations in processingconditions can alter the etch rate, the channel etching process isdifficult to control; hence the depth of the channel, which is dependenton control of the etching process, is difficult to control.

European Patent 0 500 084 A2 describes the formation of channels bypatterning of electrodes on a flat substrate, providing spacers on theflat substrate, and placing the thin glass sheet on top of the spacers.The discharge space thus extends continuously across the electrodes.However, the continuous discharge space will lead between channels tocrosstalk which is difficult to avoid. Moreover, the spacers have to beformed on the flat substrate by deposition and/or etching processes,such as screen printing. Since the spacers have to be as thick as therequired channel depth (˜100 microns or more) the fabrication of thespacers adds complexity to the process.

European Patents 0 500 085 A2 and 0554 851 A1 describe the formation ofchannels by screen printing partition walls. However, this is also adifficult process, which may require multiple coats to obtain therequired wall height.

SUMMARY OF INVENTION

An object of the invention is an improved channel plate.

A further object of the invention is an improved plasma-addresseddisplay device.

Another object of the invention is an improved method for fabricatingthe plasma channels of a PALC display device.

In accordance with a first aspect of the invention, a channel platecomprises a dielectric substrate and a thin dielectric sheet-like memberarranged over and spaced from the substrate by a plurality of laterallyspaced, channel-defining spacer members each formed as part of adielectric sheet patterned by through-holes, which latter sheet isherein referred to as the spacer sheet or plate. The holes areconfigured to form the desired channel configurations, typicallyelongated parallel channels, which preferably are straight but whichalso may be curved while still maintaining a substantially parallelrelationship. The height of the spacer sheet above the substratedetermines the height of the channels, which are each formed by theportion of the substrate surface extending between adjacent flankingspacers, the flanking spacers themselves forming the channel walls, andthe overlying portion of the thin dielectric sheet-like member. Spacedelectrodes are provided in each channel as well as a plasma-formingatmosphere. The channels are formed when the three sheet-likemembers—the substrate, the spacer plate, and the thin dielectricsheet—are assembled and bonded together.

In accordance with a second aspect of the invention, the patterning ofthe spacer sheet is such as to provide strengthening crossbars extendingpreferably transverse to and between adjacent spacer members. Thecrossbars may have a different height than that of the spacers.

In accordance with a first preferred embodiment of the invention, thesubstrate is of glass, the thin dielectric sheet is of glass, and thespacer sheet is a glass plate, with the through-holes in the form ofslots made by chemical or plasma etching or by mechanical means such assandblasting. The three glass members may be bonded together using fusedglass frit as described in several of the cited patents andpublications, or by anodic bonding as described in the first relatedpatent application identified above.

In accordance with a another preferred embodiment of the invention, thechannel plate is part of a PALC display device, and the combination ofthe substrate, patterned spacer plate and the overlying thin dielectricsheet-like member, together with the electrodes constitutes the plasmachannels or channel plate of the PALC display device.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there are illustrated and described the preferredembodiments of the invention, like reference numerals or letterssignifying the same or similar components.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic block diagram of a conventional flat panel displaysystem;

FIG. 2 is a perspective view of part of a conventional PALC displaydevice;

FIG. 3 is a perspective view of a part of one form of a channel plateaccording to the invention for use in a PALC color display, and

FIG. 4 is a top view of the spacer plate used in that channel plate;

FIG. 5 is an exploded side view of the channel plate of FIG. 3;

FIGS. 6 and 7 are schematic views indicating two different ways ofetching the spacer plate used in the embodiments of FIGS. 3-5;

FIGS. 8 and 9 are side and top views, respectively, of part of anotherform of spacer plate for use in another embodiment in accordance withthe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a flat panel display system 10, which represents a typicalPALC display device ahd the operating electronic circuitry. Withreference to FIG. 1, the flat panel display system comprises a displaypanel 12 having a display surface 14 that contains a pattern formed by arectangular planar array of nominally identical data storage or displayelements 16 mutually spaced apart by predetermined distances in thevertical and horizontal directions. Each display element 16 in the arrayrepresents the overlapping portions of thin, narrow electrodes 18arranged in vertical columns and elongate, narrow channels 20 arrangedin horizontal rows. (The electrodes 18 are hereinafter referred to fromtime to time as “column electrodes”). The display elements 16 in each ofthe rows of channels 20 represent one line of data.

The widths of column electrodes 18 and channels 20 determine thedimensions of display elements 16, which are typically of rectangularshape. Column electrodes 18 are deposited on a major surface of a firstelectrically nonconductive, optically transparent substrate 34 (FIG. 2),and the channel rows are usually built into a second transparentsubstrate 36. Skilled persons will appreciate that certain systems, suchas a reflective display of either the direct view or projection type,would require that only one substrate be optically transparent.

Column electrodes 18 receive data drive signals of the analog voltagetype developed on parallel output conductors 22′by different ones ofoutput amplifiers 23 (FIG. 2) of a data driver or drive circuit 24, andchannels 20 receive data strobe signals of the voltage pulse typedeveloped on parallel output conductors 26′by different ones of outputamplifiers 21 (FIG. 2) of a data strobe or strobe means or strobecircuit 28. Each of the channels 20 includes a reference electrode 30(FIG. 2) to which a reference potential, such as ground, common to eachchannel 20 and data strobe 28 is applied.

To synthesize an image on the entire area of display surface 14, displaysystem 10 employs a scan control circuit 32 that coordinates thefunctions of data driver 24 and data strobe 28 so that all columns ofdisplay elements 16 of display panel 12 are addressed row by row in rowscan fashion as had been described. Display panel 12 may employelectro-optic materials of different types. For example, if it uses suchmaterial that changes the polarization state of incident light rays,display panel 12 is positioned between a pair of light polarizingfilters, which cooperate with display panel 12 to change the luminanceof light propagating through them. The use of a scattering liquidcrystal cell as the electro-optic material would not require the use ofpolarizing filters, however. All such materials or layer of materialswhich attenuate transmitted or reflected light in response to thevoltage across it are referred to herein as electro-optic materials. AsLC materials are presently the most common example, the detaileddescription will refer to LC materials but it will be understood thatthe invention is not limited thereto. A color filter (not shown) may bepositioned within display panel 12 to develop multi-colored images ofcontrollable color intensity. For a projection display, color can alsobe achieved by using three separate monochrome panels 12, each of whichcontrols one primary color.

FIG. 2 illustrates the PALC version of such a flat display panel usingLC material. Only 3 of the column electrodes 18 are shown. The rowelectrodes 20 are constituted by a plurality of parallel elongatedsealed channels underlying (in FIG. 2) a layer 42 of the LC material.Each of the channels 20 is filled with an ionizable gas 44, closed offwith a thin dielectric sheet 45 typically of glass, and contains on aninterior channel surface first and second spaced elongated electrodes30, 31 which extend the full length of each channel. The first electrode30 is grounded and is commonly called the anode. The second electrode 31is called the cathode, because to it will be supplied relative to theanode electrode a negative strobe pulse sufficient to cause electrons tobe emitted from the cathode 31 to ionize the gas. As explained above,each channel 20, in turn, has its gas ionized with a strobe pulse toform a plasma and a grounded line connection to a row of pixels in theLC layer 42 above. When the strobe pulse terminates, and afterdeionization has occurred, the next channel is strobed and turned on.Since the column electrodes 18 each cross a whole column of pixels,typically only one plasma row connection at a time is allowed on toavoid crosstalk.

Fabrication of a PALC device is typically done as described in the 1993SID digest paper by providing first and second substrates 34, 36 withthe first substrate 34 comprising a glass panel on which is depositedthe ITO column electrodes 18, followed by color filter processing overthe ITO electrodes to produce the RGB stripes (not shown), followed bythe black surround processing and liquid crystal alignment processing.The second substrate 36, also a glass panel, is masked and etched toform the channels 20, following which the plasma electrode material isdeposited and masked and etched to form the cathode 31 and anode 30electrodes. A thin dielectric glass microsheet 45 is then sealed acrossthe peripheral edges of the device to form with the ridges 50 thechannels 20, which are then exhausted, back-filled with a low-pressureionizable gas such as helium and/or neon and optionally with a smallpercentage of other noble gases and sealed off. LC alignment of theexposed surface of the microsheet 45 is then carried out. The twoassembled substrates are then assembled into a panel with the two LCalignment surfaces spaced apart and facing, the LC material 42introduced into the space, and electrical connections made to the columnelectrodes 18 and plasma electrodes 30, 31.

FIG. 3 is a perspective view of part of one form of channel plate 52 inaccordance with the invention for one form of liquid crystal displaypanel in accordance with the invention. A thick flat glass bottom plate36 forms a substantially transparent dielectric substrate for the plasmachannels 20. Over the bottom plate 36 is deposited spaced electrodelayer portions 30, 31.

In accordance with the invention, the channels walls are formed in atransparent dielectric sheet 50 substantially equal in thickness to therequired channel depth. The dielectric sheet 50 is preferably of anetchable material, such as glass. This is accomplished with glass byetching through-holes 52 in the glass using conventional masking andetching processes as shown in FIG. 4.

FIGS. 6 and 7 illustrate two preferred ways for etching the glass tomake the hole walls as close to the vertical as possible. This can bedone as shown in FIG. 6 by starting with a relatively small opening 54in an etch mask 56 and etching a hole 58 whose lateral dimensions are atleast five times larger than mask opening 54 and the depth of the hole,in this case the thickness of the sheet 50. The dashed lines show theetched profiles of the sidewalls using an isotropic etchant during theetching process. It can be seen that, as the etching progresses, thesidewalls become steeper. The larger the lateral dimensions of theetched hole 58 relative to the thickness of the glass sheet 50, thesteeper the sidewalls. As an example, not meant to be limiting, for aglass sheet 50 of about 100 μm thick, to etch holes that are 500 μmwide, the mask hole 54 is preferably 100 μm wide. For a panel withstraight channels as illustrated in FIG. 2, the holes 52 would beelongated slots extending nearly the full length of the plate 50, butwould terminate at opposite sides in an annular glass border region 53so that the plate 50 remains as an integral element except for the holes52 in the form of parallel slots spaced apart by spacer walls 58.

FIG. 7 shows an etching modification in which the spacer walls can bemade even more vertical by carrying out the etching from both sides ofthe plate 50. In this case, etch masks 56 are required on both sides ofthe plate except where the holes 58 are to be formed, and the mask holes54 overlie one another. The dashed lines that show the etching profilesas the etching progresses shows that for a double-sided etching process,the sidewalls can be even steeper than with the single-sided etchingprocess shown in FIG. 6.

The thickness of the channel sidewalls 58 thus produced, it will beappreciated, represents the height of the channels 20 and constitute thespacers that space the thin dielectric sheet 45 that closes off thechannels from the substrate 36, and thus the reference to the aperturedplate 50 as the spacer plate. The etching can be by conventionalchemical etchants or by conventional plasma etching. Alternatively, amechanical erosion process can be substituted, such as sandblasting.This may be less costly and could also be used for materials for thespacer plate 50 that are more difficult to etch.

The channel electrodes 30, 31 are separately deposited and patterned onthe substrate 36 as described in the referenced papers and patents,after which the thin glass sheet 45 is attached over the aperturedspacer plate 50, and the latter then aligned and attached to thesubstrate 36 containing the electrodes. As shown, by positioning thespacer plate wails 58 over the electrodes 30, 31, then adjacent channels20 would share an electrode which would reduce the number of electrodesrequired. The walls 58 are shown with a slightly tapered shape, whichwould follow if the single-sided etching technique were used, as theglass surfaces closer to the mask hole 54 would etch more than the moreremote glass portions. If the double-sided etching process were used,then a double taper would result.

The attaching of the thin dielectric sheet 45 to the spacer plate 50,and the attaching of the spacer plate 50 to the substrate 36 can becarried out using fused glass frit at the periphery of the structure.Alternatively, anodic bonding can be used. Anodic bonding as such is awell known process for bonding two flat surfaces of ion-containingmaterials, such as glass. In a typical process, the glass sheets areplaced against each other and an electric field applied across themwhile heating them to some intermediate elevated temperature whichallows glass ions to become mobile. The ions migrate to the interfacebetween the two sheets and pulls them together. The resultant force, inthe presence of heat, leads to the formation of a permanent bond betweenthe two sheets. Typical temperatures are much lower than the softeningtemperature of the glass. The glass frit seal will only be necessary inthe electrode area after attaching the thin sheet 45.

The resultant assembled channel plate structure 52 is shown in FIG. 3.The remainder of the PALC panel can be fabricated in the usual way byfilling and sealing the plasma panel and then forming the LC part of thepanel on top of the thin glass sheet 45 as shown in FIG. 5 in anexploded view. The upper plate 34 may have deposited spacer members 60which are aligned with the spacer walls 58 and act to space apart theupper structure 34 with its ITO electrodes 18 from the glass sheet 45 toprovide a confined space for the LC material.

In a variation of the invention, where the width of the channels 20 maybe large, it may be desirable to increase the mechanical strength of thespacer plate. This can be done by etching strengthening crossbars in aspacer plate 62. This is illustrated in FIGS. 8 and 9 at 62. Thecrossbars 64 which extend laterally to and between the spacer walls 58are thus integral with the plate 62. To avoid the crossbars 64 frompossibly detrimentally affecting the operation of the plasma dischargein the channels 20, their height can be reduced without reducing theheight of the spacer walls 58. In the embodiment shown, the height h ofthe crossbars 64 is reduced from the top. The height h can be controlledby the width of the etching opening in the etch mask and the degree ofthe overetching. By appropriate masking and etching techniques, easilydetermined by those skilled in this art, the crossbars 64 can be made sothat they do not extend all the way to the height of the channels 20.

The broken lines at the edges of the elements in the figures indicatethat what is shown is a small section broken off from a larger assembly,since, as will be appreciated, typically a PALC display device formonitor use would contain several hundred column electrodes 18 andseveral hundred plasma channels 20.

It will be noted that the spacer wall portions 58 do not overlap orcover the sides of the electrically conductive layers 30, 31, which thusremain exposed and able to perform their function of igniting anelectrically conductive plasma when suitable voltages are appliedbetween them. The electrode materials are typically of a metal such ascopper, or layers of Cu—Cr—Cu, or other suitable metals.

All of the methods described in the referenced patents and publicationwill be suitable for making the remaining parts of the panel of theinvention.

The invention is generally applicable to all kinds of flat displays, andin particular to displays of the plasma-addressed type, especially PALCdisplays that typically have a small channel pitch for use in computermonitors, workstations or TV applications. While the main application ofthe channel plate of the invention is in PALC type display devices, thesame plasma plate construction 52 can also be used as a plasma displaydevice where the output is the light, generated by the plasma, which canexit the device via the transparent substrate and/or the overlyingtransparent sheet-like member.

Several preferred examples for the FIG. 3 embodiment are (all values arein μm): a wall width of about 20-50; a wall height of about 50-160; anda wall pitch of about 200-500.

It will be appreciated that the drawing figures are not to scale and inparticular the channel widths have been exaggerated to show theelectrodes.

Still further, while the channels in the substrate are typicallystraight, the invention is not limited to such a configuration and otherchannel shapes, such as a meandering shape, are also possible within thescope of the invention.

While the invention has been described in connection with preferredembodiments, it will be understood that modifications thereof within theprinciples outlined above will be evident to those skilled in the artand thus the invention is not limited to the preferred embodiments butis intended to encompass such modifications.

What is claimed is:
 1. A method for making a channel plate for a flatdisplay device, comprising: (a) providing a substantially transparentdielectric substrate, (b) providing a substantially transparent spacerplate having first and second opposed surfaces, (c) forming in thespacer plate a plurality of substantially equally-spaced walls eachhaving a height substantially equal to the thickness of the spacer platebetween the first and second surfaces, (d) bonding a thin dielectricsheet over the first surface of the spacer plate, (e) depositing anelectrically conductive material over the substrate to form on thesubstrate spaced electrodes, (f) bonding the spacer plate via its secondsurface to the substrate with the spaced walls extending generallytransverse to the substrate and such that at least portions of thespaced electrodes are located between each adjacent pair of walls. 2.The method of claim 1, wherein the substrate, the spacer plate, and thedielectric sheet are made of glass.
 3. The method of claim 1, whereinstep (c) is carried out by chemically or plasma etching slots in thespacer plate.
 4. The method of claim 1, wherein step (c) is carried outby chemically etching the spacer plate using an etch mask with openingshaving a smaller width than the spacing between the spacer plate walls.5. The method of claim 1, wherein step (c) is carried out bymechanically eroding slots between the spacer plate walls.
 6. The methodof claim 1, wherein spaced crossbars are formed in the spacer plate soas to extend laterally between flanking wall portions.
 7. The method ofclaim 6, wherein the crossbars have a smaller height than that of theflanking wall portions.
 8. The method of claim 1, wherein the bondingsteps are carried out by glass frit or anodic bonding.
 9. A method formaking a plasma channel plate of a plasma-addressed electro-opticdisplay device comprising a layer of electro-optic material, dataelectrodes coupled to the electro-optic layer and adapted to receivedata voltages for activating portions of the electro-optic layer, aplurality of elongated plasma channels extending generally transverse tothe data electrodes for selectively switching on said electro-opticportions, and a dielectric sheet closing off the plasma channels on theside facing the data electrodes, said plasma channels each comprisingspaced elongated cathode and anode plasma electrodes and an ionizablegas filling said plasma channels, said method comprising: (a) providinga substantially transparent dielectric substrate, (b) providing asubstantially transparent dielectric spacer plate having first andsecond opposed surfaces, (c) forming by etching in the spacer plate aplurality of substantially equally-spaced walls each having a heightsubstantially equal to the thickness of the spacer plate between thefirst and second surfaces, (d) bonding a dielectric sheet over the firstsurface of the spacer plate, (e) depositing an electrically conductivematerial over the substrate to form on the substrate a plurality ofspaced cathode and anode electrodes, (f) bonding the spacer plate viaits second surface to the substrate with the spaced walls extendinggenerally transverse to the substrate and such that a pair of the spacedcathode and anode electrodes are located between each adjacent pair ofwalls.
 10. The method for making the plasma channel plate of aplasma-addressed electro-optic display device as claimed in claim 9,wherein the cathode and anode electrodes are wider than the spaced wallsand the spaced walls are located over the cathode and anode electrodessuch that adjacent channels share an electrode.
 11. The method of claim1, wherein the electrodes are wider than the spaced walls and the spacedwalls are located over the electrodes such that adjacent channels sharean electrode.
 12. The method of claim 1, wherein the electrodes comprisecathode and anode electrodes formed on the substrate.